The present invention relates to a row conductor drive circuit for a matrix display device, and in particular to a row conductor drive circuit for an "active matrix" type of matrix display device in which each picture element of the display is controlled by an individual control element, i.e. a thin-film transistor, and in which the row conductor drive circuit is formed directly upon the same substrate of a display panel of the matrix display device as these control transistors.
The "active matrix" type of matrix display device is becoming widely used, generally as a liquid crystal type of display device, and has shown suitability for providing matrix displays of extremely small size, such as are required for wrist-watch television receivers. With such a display device, a plurality of row conductors and a plurality of column conductors are formed upon a panel of the display, insulated from one another at their intersections, with a transistor coupled to a capacitor being formed at each of these intersections. The gate electrode of each transistor is coupled to the corresponding row conductor, and when that row conductor is selected by being set to a predetermined potential by a row scanning signal, a signal level representing picture data is transferred through the transistor and stored as a charge on the capacitor. The resultant capacitor potential determines the visual state of a corresponding picture element.
In the case of such a very small size of display device, greater compactness can be achieved by forming the row drive circuit (i.e. the circuit which generates row scanning signals to successively select the row conductors of the matrix) directly upon the same display panel as that on which the control transistors and row conductors and column conductors are formed. Such a row conductor drive circuit generally comprises a shift register, with each shift register stage output being coupled to a corresponding one of the row conductors, and with the shift register outputs successively producing the row scanning signals (e.g. in the case of a television display, scanning all of the row conductors during each period of the vertical sync pulse signal).
A problem which arises with regard to the practical manufacture of a matrix display device provided with such a row conductor drive circuit is that if a single shift register stage should be defective, then the entire row conductor drive circuit (and hence the entire matrix display device) is unusable. As a result of this, the manufacturing yield is comparatively low, thereby tending to increase the cost of manufacture of such a matrix display device.
One method which has been proposed in the prior art, and proposed in Japanese Pat. No. 56-104388, is to utilize a pair of shift register effectively connected in parallel to the row conductors, i.e. respectively formed on the right hand and left hand sides of the display panel, and select one of these shift registers to be used as the row conductor drive circuit after it has been confirmed to be functioning correctly. Such a method will enable a certain degree of improvement in the manufacturing yield, but since it is still necessary that at least one of these two shift registers (each made up of, for example, 210 stages connected in series) must be operating correctly, only a relatively small improvement in the yield can be expected.
There is therefore a requirement for a row conductor drive circuit for a matrix display device of the type described above, which will enable a substantially greater improvement to be made in the manufacturing yield of such devices, and to thereby render their application more practical.